The present invention relates generally to a radar tool for use in a borehole for locating fractures in a geological formation near the borehole, and more specifically to such a tool having directional transmitting and receiving antennas including barium titanate to couple the antenna to the formation.
A "nonproductive" dry hole constitutes a significant expense in geothermal and petroleum exploration. Accordingly, much research has been undertaken to enable the accurate prediction of subsurface conditions prior to drilling. For example, surface geophysical surveys may be conducted prior to drilling to determine the probability that a reservoir exists in an area. However, the low frequencies used in these surface surveys have limited resolution and cannot identify fine structures such as millimeter-thick fractures in the formation that enable fluids to move through the formation.
Downhole instruments provide more detailed information on the subsurface structure than surface techniques. It is known that an electromagnetic (EM) wave in the 30 MHz-300 MHz range can propagate tens of meters into a formation and provide detectable returns from a fracture. However, the small diameter of a borehole has prevented the use of a directional antenna at the preferred frequency range in the azimuthal angle (on a plane perpendicular to the wellbore). Accordingly, prior measurements have relied on a transmitter and a receiver in different wellbores, to obtain information only about the plane connecting the wellbores. A three dimensional view of an underground formation would require many wellbores, an expensive proposition.
Radar systems with directional antennas that fit a typical wellbore have been developed for frequencies greater than 300 MHz. Unfortunately, the depth of penetration of EM waves at such frequencies is limited. However, a directional system at 30 MHz to 300 MHz would have greater penetration and, therefore, such a system could be used, for example, to guide a drill from a nonproductive dry hole towards nearby fractures.